It is well known in the art how to generate medium BTU gas from coal in above ground gasifiers. For this purpose a particular type of coal is selected so that the above ground gasifier will not become clogged during the process. The coal is mined, transported from the mine to the gasifier site, crushed to the proper lump size, then charged into the gasifier which is operated at a pressure above atmospheric. Since the gasifier is pressurized, suitable mechanical pressure locking chambers must be employed in order to feed the coal in steps from atmospheric pressure to the operative pressure required. The coal is then burned with oxygen and the ash is collected in mechanical pressure locking chambers so that the ash may be removed at atmospheric pressure. The gasifier itself is primarily a pressure vessel made of metal parts, and it is necessary to control combustion temperatures so that metal parts are not damaged. Generally it is desirable to control temperatures below that of the fusion temperature of the ash so that the ash may be removed as a dry solid rather than in molten form. Temperature control is normally provided by injecting steam along with the oxygen into the gasifier, with ratios of steam injected to coal consumed in the order of pound for pound. In this manner medium BTU gas, in the range of 400 to 600 BTUs per standard cubic foot, is generated.
In the production of coal in situ in some cases it may be desirable to control underground combustion temperatures below the fusion point temperature of the ash in order to keep the ash from flowing underground in molten form. In situ production of coal requires no metal parts in the reaction zone, therefore temperature control to protect metal parts is not needed. Thus less steam is required for temperature control while generating a medium BTU gas. Further, the ash is left underground rather than creating the disposal problem which is inherent in above ground gasifiers.
Generally the prior art methods for production of coal in situ do not provide for temperature limits in the underground reaction zone. The use of steam in alternate cycles is taught in U.S. Pat. No. 4,018,481 of the present inventor. Another use of steam is taught in U.S. Pat. No. 3,794,116 of Higgins wherein it is necessary first to rubblize the underground coal.
It is well known in the art how to fire projectiles underground to establish communications between a well bore and producing horizon such as an oil saturated sand stratum. In this case a perforating gun is lowered into a well bore opposite the oil bearing stratum, and multiple shots are fired with the projectiles penetrating the well casing, the cement between the well casing and the well bore, and into the oil sand until the momentum of the projectile is spent. In this manner openings are created in the casing and cement, and channels are formed in the oil sand. Such channels may have a length of a few inches and in some cases as much as 10 feet. The object of such channels to provide free flowing communications passages through the underground oil sand, particularly in the immediate vicinity of the well bore which may have become impervious to the passage of fluids due to invasion of drilling mud during the drilling operations.
It is well known in the art how to produce coal in situ using vertical and linked wells. Two or more wells are bored from the surface of the ground into the coal deposit. Compressed oxidizer is injected into one well and eventually a portion of the oxidizer will reach the second well, at which time the coal in the second well is ignited. By continuing injection of oxidizer in the first well, the fire will propagate through the coal toward the on coming oxygen and will eventually burn a channel linking the two wells underground.
It is common in underground coal deposits that a system of cracks is found within the coal. These cracks, sometimes called cleats, form a general geometric pattern with one series of cleats being generally perpendicular to the other series of cleats that traverses the coal deposit. The coal itself generally has very low permeability for the passage of fluids, but often one series of cleats will have a considerable amount of permeability with 300 millidarcies not being uncommon. The preponderance of the oxidizer passing through the coal seam, as heretofore mentioned, proceeds from one well to the next through the series of cleats in the coal.
The oxidizer under the influence of differential pressure proceeds primarily through paths of least resistance through the coal seam. The path through the coal seam carrying the maximum oxidizer flow will be the path of the channel when two wells are linked by an underground burn. Such a path generally is quite circuitous in its traverse and may deviate substantially from a straight line drawn between the two wells. The pattern of wells drilled for in situ production of coal generally conforms to a predetermined geometric pattern such as a series of rows of wells in parallel with each other. Significant meanderings of the underground channels burned in the coal tend to render ineffective any preplanned well pattern. Therefore it is desirable to burn underground channels with minimum deviations from straight lines in order to assure that large portions of the underground coal will not be bypassed as the in situ processes proceed.
It is an object of the present invention to teach the control of temperatures in the underground reaction zone while generating a medium BTU gas. It is another object of the present invention to teach methods of burning underground channels through a coal seam with minimum deviations from the planned directions for such channels. Other objects, capabilities and advantages of the present invention will become apparent as the description proceeds.